Material Damage System and Method for Determining Same

ABSTRACT

A system and method for determining a change in a thickness and temperature of a surface of a material are disclosed herein. The system and the method are usable in a thermal protection system of a space vehicle, such as an aeroshell of a space vehicle. The system and method may incorporate micro electric sensors arranged in a ladder network and capacitor strip sensors. Corrosion or ablation causes a change in an electrical property of the sensors. An amount of or rate of the corrosion or the ablation and a temperature of the material is determined based on the change of the electrical property of the sensors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application claiming the benefit ofpriority from U.S. patent application Ser. No. 13/301,249, entitled“Material Damage System and Method for Determining Same”, filed on Nov.21, 2011, pending, each of which is hereby incorporated by reference inits entirety.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment for Government purposes without the payment of any royaltiesthereon or therefore.

BACKGROUND OF THE DISCLOSURE

Damage to materials due to use and environmental conditions isproblematic in many industries. Corrosion, ablation and erosion areexamples of material damage that effect industrial applicability and useresulting in increased maintenance costs, compromised safety, higherproduction costs and other negative results. In order to mitigate thisdamage, corrosion monitoring is necessary.

In the aeronautical industry, for example, thermal protection systemsare critical for the protection of space vehicles and payloads duringre-entry. The thermal protection system is usually attached to theentire front surface of the aero-shell that bears the major blunt ofatmospheric re-entry. The mission success of the space vehicle isfundamentally dependent on the thermal protection system materialprotecting the aero-shell from the aggressive conditions encounteredduring entry. Several entry conditions (e.g. radiation, shock andionization) combine to ablate the thermal protection system material andtheir effects increase as the anticipated size and mass of futurevehicles destined for planets with atmosphere increases. It is,therefore, important to determine the temperature and rate at which thethermal protection system material recedes toward the aero-shell due toablation.

The current state of the art of instrumentation of the thermalprotection system uses conventional thermocouples and resistors to sensetemperature and resistance, respectively. These thermocouples andresistors are manually placed in cylindrical plugs that are made fromthe temperature protection system material. The plugs are subsequentlyinserted in holes drilled in the main thermal protection system materialthat is incorporated into the aero-shell. The purpose of thethermocouples is to measure the temperature spatial and temporaltemperature gradient along the trajectory axis of the thermal protectionsystem material and also over the surface of the thermal protectionsystem. The resistor measures the ablation of the char layer of thethermal protections system material. The sensors are embedded in theplug with the thermocouple and then inserted into the main thermalprotection system material.

There are several problems with these sensor arrangements. First, theseinstrumented plugs are time consuming to manufacture and problematic tointegrate into a space vehicle. Integration requires machining holes toaccommodate these instrumented cylindrical plugs. The cost and time tointegrate plugs into a heat shield of the space vehicle can havesignificant cost and schedule impacts. Presently, insertion and gluingof the plug into the thermal protection system material leaves acircular boundary of homogeneous material discontinuity between the plugand the main thermal protection system. The circular boundary is definedby the glue material. During entry phase, the boundary layershock/thermal protection system interaction could preferentially ablatethis circular boundary section, leading to enhanced turbulence andaccelerated ablation. The potential result would be the disgorging ofthe plug and exposure of the aero-shell, thus compromising the safety ofthe vehicle. To dramatically reduce the impact of incorporatinginstrumentation into a thermal protection system of the space vehicle, anew measurement system and methodology is needed.

Second, the plug approach limits the number of thermocouple and resistorcarrying plugs that can be positioned in the thermal protection system.Too many plugs, for the purpose of improving area coverage andresolution, could potentially affect the structural and mechanicalintegrity of the thermal protection system material. It could alsoincrease the number of possible sites for shock induced damage. Also,the manual arrangement of the sensors within the plug severely reducesthe number of sensors needed for high resolution profiling of thetemperature gradient and the ablation recession rate. Additionally, dueto the limited number of sensors, a high resolution, large areatomographic profile of the thermal protection system is impossible toobtain.

It is anticipated that the thermal protection system area will continueto increase with increasing payload, a tomographic profile of thecondition of the thermal protection system becomes important inmonitoring entry and actively changing the entry axis to avoid risks.Accordingly, to improve thermal protection systems of space vehicles aswell as detecting damage to materials of other surfaces, whether causedby corrosion, ablation or the like, a new system and method of use isneeded.

SUMMARY OF THE DISCLOSURE

In an embodiment of the disclosure, a system is disclosed having amaterial susceptible to corrosion or ablation and having a thicknessdefined between a first end and a second end. The first sensor and asecond sensor are on the material and extend along the thicknessadjacent the first end and toward the second end. The first sensor hasan electrical component that changes an electrical property of thesensor as temperature of the material changes. The second sensor has anelectrical property that is substantially constant with respect tochanges in temperature of the material. An electrical circuit coupled tothe first sensor and the second sensor determines the change in theelectrical property of each of the first sensor and the second sensor. Aprocessor determines a thickness and a temperature of the material basedon the change of the electrical property of the first sensor and thesecond sensor.

In another embodiment of the disclosure, a method is disclosedpositioning a first micro electric sensor and a second micro electricsensor within a susceptible material. The method further includesapplying a current or voltage to the first micro electric sensor and thesecond micro electric sensor. Furthermore, the method includes exposingthe susceptible material to a substance causing a change in atemperature and thickness of the susceptible material. In addition, themethod includes determining the temperature of the susceptible materialfrom an electrical property of the first micro electric sensor.Moreover, the method includes determining the thickness of thesusceptible material from an electrical property of the second microelectric sensor. The electrical property of the first micro electricsensor changes substantially more with temperature than the second microelectric sensor.

In yet another embodiment of the disclosure, a method is disclosed forembedding a plurality of sensors within a material. The plurality ofsensors have at least a first ladder sensor, a second ladder sensor, anda capacitor strip. The second ladder sensor has more rungs than thefirst ladder sensor. Further, the second ladder sensor has electricalcomponents that are less temperature sensitive than electricalcomponents of the first ladder sensor. The method also includes applyinga current or voltage to each of the plurality of sensors, and exposingthe material to a change in temperature causing a change in anelectrical property of at least the second ladder sensor. Furthermore,the method includes ablating a portion of the material to change anelectrical property of at least the first ladder sensor and thecapacitor strip. Still further, the method includes determining atemperature of the material based on the change in the electricalproperty of the first ladder sensor and the second ladder sensor.Moreover, the method includes determining a thickness of the materialbased on the change of the electrical property of the capacitor stripand the change of the electrical property of the second ladder sensor.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a plurality of sensors on a material in an embodimentof the present disclosure.

FIG. 2 illustrates a sensor in an embodiment of the disclosure.

FIG. 3 illustrates a sensor connected to a sensing device, source andprocessor in an embodiment of the disclosure.

FIG. 4 illustrates a first sensor and a second sensor connected to aprocessor in an embodiment of the disclosure.

FIG. 5 illustrates a plurality of sensors within a material in anembodiment of the present disclosure.

FIG. 6A illustrates a first sensor, a second sensor, and a third sensoron a material in another embodiment of the present disclosure.

FIG. 6B illustrates a cross-sectional view of the first sensor along theline A-A as shown in FIG. 6A.

FIG. 6C illustrates a cross-sectional view of the second sensor alongthe line A-A as shown in FIG. 6A.

FIG. 6D illustrates a cross-sectional view of the third sensor along theline A-A as shown in FIG. 6A.

FIGS. 7A-7G illustrate embodiments of steps of manufacturing one or moresensors in an embodiment of the disclosure.

FIGS. 8A-8D illustrate embodiments of additional steps of manufacturingone or more sensors in an embodiment of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

Embodiments of the present disclosure generally describe a system andmethod for determining temperature and/or thickness of a surface of asusceptible material. The susceptible material may be susceptible to achange in thickness by exposure to extreme temperatures, radiactivesources, corrosive substances, or other sources that may damage thesurface of and/or structure of the susceptible material. Whileembodiments of the present disclosure are described as being implementedwith a sensor having a ladder network, the present disclosure should notinterpreted as limited as requiring the use of a ladder network. Thepresent disclosure describes sensors that may be implemented within thespirit of the invention that are not arranged in a ladder network.

In addition, the present disclosure should not be deemed as limited touse on a space vehicle or in the aerospace industry. Those havingordinary skill in the art will appreciate that the present disclosurehas various applications in many industries. For example, theembodiments disclosed herein have broad applications across any industrywhere it is desirous to determine corrosion, erosion, ablation,recession, and surface temperature of a material.

In at least some embodiments, the present disclosure providestemperature and recession rate measurements of a susceptible material.For example, the present disclosure may utilize or employ a sensorcomprising an electrical circuit having components the cause a change inan electrical property upon corrosion, ablation or a change intemperature of the susceptible material.

FIG. 1 illustrates an embodiment of a system 100 comprised of aplurality of sensors 10 and a plurality of sensors 15 a-15 e arranged ona substrate 12. The system 100 may utilize the substrate 12 in order tofabricate the sensors 10, 15 a-15 e. For example, the system 100 may bea micro electromechanical system (“MEMS”) whereby the sensors 10, 15a-15 e are micro electric sensors. In such an embodiment, the substrate12 may be used to fabricate one or more of the sensors 10, 15 a-15 e.

As one example of fabricating the sensors 10, 15 a-15 e, the substrate12 may be selectively removed in order to realize miniaturizedcomponents. Such micromachining may be accomplished using chemical orphysical means. For example, a bulk micromachining technique may beused, such as chemical wet etching that involves immersion of asubstrate, such as the substrate 12, into a reactive chemical solution.As a result of immersion in the reactive chemical solution, the exposedregions of the substrate 12 may be etched at measurable rates. Chemicalwet etching may be used to provide a relatively high etch rate andpermit selective etching.

Surface micromachining is another method that may be used for thefabrication sensors 10, 15 a-15 e on the substrate 12 in an embodimentwhere the sensors 10, 15 a-15 e are micro electric devices or MEMSdevices. Generally, surface micromachining may involve deposition of afilm material to act as a temporary mechanical layer onto which theactual device layers are built. Next, a structural layer may bedeposited and patterned on the film material followed by the removal ofthe temporary layer to release the mechanical structure layer from theconstraint of the underlying layer. As a result, the structural layermay be freed. One of the reasons surface micromachining may be used isthat it provides for precise dimensional control.

A person having ordinary skill in the art will appreciate many methodsof fabricating the sensors 10, 15 a-15 e. The invention should not bedeemed as limited to any specific fabrication technique. In addition,the sensors 10, 15 a-15 e being micro electric sensors and/or MEMSsensors are an embodiment and the disclosure is not limited thereto.

As shown in FIG. 1, each of the sensors 10 may have a plurality ofelectrical components 14 a-14 i, 16 a-16 h, and 18 a-18 h. One or moreof the sensors 15 a-15 h may be positioned adjacent each of the sensors10. In an embodiment, the sensors 15 a-15 h each have a capacitor stripconfigured to ablate, recess or otherwise have its length change inresponse to ablation, recession, corrosion or erosion of the thicknessof the substrate 12. In such an embodiment, the capacitor strip maycomprise a pair of opposing electrodes, a pair of electric wires, acoaxial cable, micro sized capacitors positioned in series or inparallel, a single wire, a pair of opposing plates, or any type ofcapacitive device that will change its capacitance with length orsurface area. The sensors 15 a-15 h may not be temperature dependentsuch that any change in the temperature of the substrate 12 will havesubstantially zero effect on electrical properties of the sensors 15a-15 h. Accordingly, the sensors 15 a-15 h may be positioned into adevice to measure changes in dimension, such as changes in a thicknessof the substrate 12.

FIG. 2 illustrates the sensor 10 having the plurality of electricalcomponents 14 a-14 i, 16 a-16 h, and 18 a-18 h that may be used in thesystem 100. The electrical components 14 a-14 i, 16 a-16 h, and 18 a-18h of the sensor 10 may be in electrical communication to form anelectrical network. The electrical components 14 a-14 i, 16 a-16 h, and18 a-18 h may comprise conducting lines, conducting wires, resistors,and/or inductors. The electrical components 14 a-14 i, 16 a-16 h, and 18a-18 h may be in parallel or in series as shown in FIG. 2. In anembodiment, each of the electrical components 14 a-14 i, 16 a-16 h, and18 a-18 h has at least one resistor or inductor. The electricalcomponents 14 a-14 i, 16 a-16 h, and 18 a-18 h may have at least oneresistor, or inductor in addition to or alternative to a conducting lineor wire.

The arrangement of the electrical components 14 a-14 i, 16 a-16 h, and18 a-18 h may be in a ladder network configuration as shown in FIG. 2.The ladder network configuration may permit a portion of the electricalcomponents 14 a-14 i, 16 a-16 h, and 18 a-18 h to become electricallyseparated while permitting electrical communication between the otherportion of the electrical components 14 a-14 i, 16 a-16 h, and 18 a-18 hto terminals 20 a, 20 b, as shown in FIG. 2. As an example, theelectrical component 14 a may be removed, may be destroyed, or mayotherwise be electrically separated from the other electrical components14 b-14 i, 16 a-16 h, and 18 a-18 h. Electrically disconnecting theelectrical component 14 a, for example, may change a resistance, or acurrent in the sensor 10. Conversely, a break, ablation or the like inthe sensor 15 may change the capacitance to correspond with a newthickness of the eroding material.

As mentioned, the electrical components 14 a-14 i, 16 a-16 h, and 18a-18 h may be electrically connected to the terminals 20 a, 20 b, asshown in FIG. 2. As a portion of the electrical components 14 a-14 i, 16a-16 h, and 18 a-18 h are electrically separated from the electricalnetwork, the resistance or the current as measurable from the terminals20 a, 20 b may change.

In addition, the electrical components 14 a-14 i, 16 a-16 h, and 18 a-18h may be temperature sensitive such that a change in a temperature of atleast one of the electrical components 14 a-14 i, 16 a-16 h, and 18 a-18h causes a change in the electrical property of the sensor 10. Theelectrical components 14 a-14 i, 16 a-16 h, and 18 a-18 h may have apositive or negative temperature coefficient. In the case of a positivetemperature coefficient, a resistance or capacitance of one or more ofthe electrical components 14 a-14 i, 16 a-16 h, or 18 a-18 h, mayincrease with an increase in temperature. On the other hand, a negativetemperature coefficient results in a decrease in resistance orcapacitance as temperature increases. It should also be noted that azero temperature coefficient is also a possible coefficient of theelectrical components 14 a-14 i, 16 a-16 h, and 18 a-18 h.

FIG. 3 illustrates an embodiment of the sensor 10 and the sensors 15 a,15 b. For explanatory purposes, the electrical components 14 a-14 f, 16a-16 f, and 18 a-18 f are shown as each having a resistor. Theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f are connectedto the terminals 20 a, 20 b and arranged in a ladder network. Theelectrical component 16 f is electrically connected to the terminal 20 aand may extend substantially parallel to the electrical component 18 f,which is electrically connected to the terminal 20 b. The electricalcomponent 14 f is electrically connected to the electrical components 16f, 18 f. The electrical component 14 f may be substantiallyperpendicular to the electrical components 16 f, 18 f. The electricalcomponents 16 e and 18 e may be electrically connected to the electricalcomponents 14 f, 16 f, 18 f and extend away from the terminals 20 a, 20b. The other electrical components 14 a-14 e, 16 a-16 e, and 18 a-18 dmay be electrically connected in a similar manner as shown in FIG. 3.

The sensors 15 a, 15 b extend in a direction substantially parallel tothe sensor 10. The sensors 15 a, 15 b may, in an embodiment, eachcomprise a wire, a semiconductor, a doped ceramic, or a plate forstoring a charge. In another embodiment, each of the sensors 15 a, 15 bmay comprise a pair of wires, a pair of plates or one or more devicespositioned along its length for storing a charge. The sensors 15 a, 15 bmay be configured such that a change in its length will change anelectrical property of the sensors 15 a, 15 b. The change in theelectrical property may be related to a change in the length of sensors15 a, 15 b.

An electric circuit 50 is connected to the sensor 10 via the terminals20 a, 20 b. The electrical circuit 50 is electrically connected to theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f, via theterminals 20 a, 20 b. For example, as shown in FIGS. 4 and 5, conductinglines 30 a, 30 b may extend from each of the sensors 10 out of thesecond end 8 of the susceptible material 42 and connect to the terminals20 a, 20 b. The electrical circuit 50 may be electrically connected tothe sensor 10 via the terminals 20 a, 20 b. The electrical circuit 50may also be connected to the sensors 15 a, 15 b via terminals 21 a, 21b. Several other alternative series/parallel connections exist, but theabove is for illustrative purposes.

The electrical circuit 50 may comprise an electrical energy source 22electrically connected to the terminals 20 a, 20 b and/or the terminals21 a, 21 b. The electrical energy source 22 may deliver a current and/ora voltage to the sensor 10 and/or the sensors 15 a, 15 b. For example,the electrical energy source 22 may deliver a constant voltage or aconstant current to the sensor 10, such as the electrical components 14a-14 f, 16 a-16 f, and 18 a-18 f, via the terminals 20 a, 20 b. In thecase of constant voltage, a change in a resistance of one of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f results in achange in the current passing through the electric circuit 50. Anincrease in a resistance, results in a lower current, while a decreasein resistance results in an increased current. The electrical energysource 22 may also deliver a voltage or current to the sensors 15 a, 15b. In an embodiment where the sensors 15 a, 15 b are capacitors, achange in length of the sensors 15 a, 15 b may change an electricalproperty of the sensors 15 a, 15 b. For example, as the length of thesensors 15 a, 15 b decrease, a capacitance of the sensors 15 a, 15 b maydecrease.

The electrical circuit 50 may have a measuring device 24 to measure anelectrical property of the electrical circuit 50, the sensor 10 and/orthe sensors 15 a, 15 b. The measuring device 24 may be positionedbetween the terminals 20 a, 20 b. The measuring device 24 or a secondmeasuring device (not shown) may be in electrical communication with theterminals 21 a, 21 b of the sensors 15 a, 15 b. The measuring device 24may determine and/or may measure a current or voltage of the sensor 10and the sensors 15 a, 15 b. In an embodiment, the measuring device 24may be an ammeter, or digital meter, to measure and/or determine anamount of current passing therethrough. The measuring device 24 mayinstead or in addition to current measure voltage.

A processor 26 may be electrically connected to the measuring device 24to receive measurement data from the measuring device 24. FIG. 4illustrates an embodiment of the processor 26 connected to the measuringdevice 24 of each of the sensors 10. The processor 26 may also be incommunication with terminals 21 a, 21 b of the sensors 15 a, 15 b. Inthis embodiment, the terminals 21 a, 21 b may contain and/or mayincorporate a measuring device for determining an electrical property ofthe sensors 15 a, 15 b, such as current, voltage or capacitance. Theterminals 21 a, 21 b may communicate the electrical property to theprocessor 26 for determination of a length of the sensors 15 a, 15 b.The processor 26 may be positioned locally or remotely with respect tothe electrical circuit 50 and the sensor 10.

FIG. 4 also illustrates the sensors 10, 15 a, 15 b positioned between afirst end 6 and a second end 8 of a susceptible material 42. Thesusceptible material 42 may be any material or device in whichcorrosion, ablation, erosion and/or surface temperature determinationmay be advantageous. Non-limiting examples of the susceptible material42 include a braking material for a vehicle, a thermal protection systemof a space vehicle, or a fluid conduit. A circuit method of multiplexingcan be used to connect numerous sensors 10, 15 a, 15 b so that sampling,such as high speed sampling, may be used to interrogate a plurality ofthe sensors 10, 15 a, 15 b to allow for a real-time, or substantiallyreal-time, topographic display of the erosion on a display (not shown).

The thickness of the susceptible material 42 may be defined between thefirst end 6 and the second end 8. The first end 6 of the susceptiblematerial 42 may be positioned adjacent to and/or exposed to a substancethat may cause a change in the thickness of the susceptible material 42.For example, the substance causing a change in thickness of thesusceptible material 42 may be a fluid at an extreme temperature, aradiactive substance, a corrosive substance, or other substance that maycause a change in the thickness of the susceptible material, 42, such asby damage to the surface of and/or structure of the susceptible material42.

The substance may change the thickness of the susceptible material 42causing separation of a portion of the sensor 10 from a remainingportion of the sensor 10 and a change in length of the sensors 15 a, 15b. For example, the substance may decrease the thickness of thesusceptible material 42 and electrically separate one or more of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f, such as theelectrical component 14 a, from the sensor 10. Additionally, thesubstance may decrease the thickness of the susceptible material 42 andelectrically separate a portion of the sensors 15 a, 15 b and/ordecrease a length of the sensors 15 a, 15 b. As a result, the measuringdevice 24 may measure a change in the electrical property of theelectrical circuit 50, such as a change in the current. In anembodiment, upon electrical separation of the electrical component 14 a,the current measured at the measuring device 24 may decrease due to anincreased resistance of the electrical circuit 50, if, for example, theelectrical component 14 a is a resistor. If, for example, the sensors 15a, 15 b are capacitor strips, a decrease in length of the sensors 15 a,15 b may decrease a capacitance. The processor 26 may be in electricaland/or data communication with the measuring device 24 to relate achange in the electrical property of the sensor 10 and the sensors 15 a,15 b to a change in the thickness of the susceptible material 42. In anembodiment, the processor 26 may compare a computed change in thicknessof the susceptible material 42 from the sensors 10 with a computedchange in thickness of the susceptible material 42 from the sensors 15a, 15 b.

Turning again to FIG. 4, the substance changing the thickness of thesusceptible material 42 may cause a greater change to the susceptiblematerial 42 at one of the sensors 10, 15 a, 15 b than one of the othersensors 10, 15 a, 15 b. In such an event, the processor 26 may receivedata and/or communication from the measuring device 24 and determine thethickness of the susceptible material 42 at each of the sensors 10, 15a, 15 b. Advantageously, the sensors 10, 15 a, 15 b may be spaced atpredetermined locations to measure a change of thickness, such as due tocorrosion or ablation. While FIG. 4 only illustrates two of the sensors10, any number of the sensors 10 may be positioned at predeterminedlocations along the susceptible material 42 resulting in informationrelated to the thickness at each location of each of the sensors 10.

In addition to the change of thickness of the susceptible material 42,the sensor 10 may determine a temperature of the susceptible material42. The change in temperature of the susceptible material 42 may cause achange in temperature of one or more of the electrical components 14a-14 f, 16 a-16 f, and 18 a-18 f. As the temperature of one of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f changes, anelectrical property of the sensor 10 changes. For example, the electricproperty of the sensor 10 may be a resistance or capacitance that maychange with temperature. The change in the capacitance of resistance ofthe sensor 10 may be the result of the change in temperature. Changingthe electrical property may result in a change in current passingthrough the sensor 10 and/or current at the measuring device 24.

As set forth above, the change in the temperature and the change in thethickness of the susceptible material 42 may cause a change in theelectrical property of the sensor 10. The processor 26 may determine thechange in the electrical property due to the temperature and the changein the electrical property due to the change in thickness of thesusceptible material 42. The processor 26 may have information and/ormay determine a relationship of the electrical property of each of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f andtemperature. In addition, the processor 26 may have information and/ormay determine a relationship of the electrical property of each of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f and a changein the thickness of the susceptible material 42. The measuring device 24and/or the processor 26 may determine a change in the electricalproperty of the sensor 10. As mentioned, the change in the electricalproperty of the sensor 10 may be a change in current, voltage or thelike. The processor 26 may then determine the change in electricalproperty due to the temperature change and the change in the electoralproperty due to the change in thickness of the susceptible material 42.

As in the example of FIG. 4, the electrical components 14 a-14 f, 16a-16 f, and 18 a-18 f may be resistors. If these resistors have anegative temperature coefficient, an increase in temperature of theelectrical components 14 a-14 f, 16 a-16 f, and 18 a-18 f may cause adecrease in resistance. Simultaneously if the increase in temperaturecauses a decrease of thickness of the susceptible material 42 byablating a portion of the susceptible material 42 and the electricalcomponent 14 a, then the resistance of the sensor 10 may increase. Themeasuring device 24 may measure a change in current as a result of thechange in resistance. For explanatory purposes, if the overallresistance increases, then the current at the measuring device 24 maydecrease. However, the current at the measuring device 24 may be higherthan expected if the electrical component 14 a is separated fromelectrical communication with the other electrical components 14 b-14 f,16 a-16 f, and 18 a-18 f. The current may be higher than expected as thetemperature increase may cause the resistance in the other electricalcomponents 14 b-14 f, 16 a-16 f, and 18 a-18 f to decrease. Theprocessor 26 may determine the change of thickness of the susceptiblematerial 42 and the change in temperature of the susceptible material 42due to the decrease in the current.

As another example, the sensors 10 as shown in FIG. 4 may be arrangedsuch that any change in thickness of the susceptible material 42 at oneof the sensors 10 will cause the same or a substantially similar changein thickness of the susceptible material 42 at the other sensor 10. Inorder to separate the change in electrical property due to temperaturefrom the change in electrical property due to the change in thickness,one of the sensors 10 may have the electrical components 14 a-14 f, 16a-16 f, and 18 a-18 f that do not change with temperature. In yetanother embodiment, one of the sensors 10 may have the electricalcomponents 14 a-14 f, 16 a-16 f, and 18 a-18 f with positive temperaturecoefficients and the other sensor 10 may have the electrical components14 a-14 f, 16 a-16 f, and 18 a-18 f with negative temperaturecoefficients. The electrical components 14 a-14 f, 16 a-16 f, and 18a-18 f of the sensors 10 may have the same number of resistors,capacitors and electrical conductors such that the sensors 10 aresimilar. Alternatively, the sensors 10 may have a distinct number, type,location and/or arrangement of the electrical components 14 a-14 f, 16a-16 f, and 18 a-18 f. In either case, the processor 26 may utilize thisinformation to determine a change in thickness of the susceptiblematerial 42 and/or a temperature of the susceptible material 42. Theseembodiments are not mutually exclusive and may be used in combination inorder to determine temperature and thickness of the susceptible material42 at each of the sensors 10.

In an embodiment, the sensors 15 a, 15 b may be used to determine achange in thickness of the susceptible material 42. In order to ensureaccurate measurement of the change in thickness, the sensors 15 a, 15 bmay not vary with temperature. Accordingly, the entire change, or atleast a substantial amount of the change, in electrical property of thesensors 15 a, 15 b may be attributed to a change in thickness of thesusceptible material 42. Again, the processor 26 may determine thechange in thickness of the susceptible material 42 based on the changein the electrical property of the sensors 15 a, 15 b. The temperature ofthe susceptible material 42 may be determined from the sensors 10, suchas by the processor 26 computing the temperature based on a change inthe electrical property of the sensor 10. Accordingly, a temperature andthickness of the susceptible material 42 may be determined. Thetemperature and thickness of the susceptible material 42 may bedetermined along an entire surface to generate a tomographic image ofthe susceptible material 42.

In addition to utilizing the sensor 10 to determine the temperature ofthe susceptible material 42, the change in thickness of the susceptiblematerial 42 may be determined from a change in electrical property ofthe sensor 10. The processor 26 may determine the change in thickness ofthe susceptible material 42 from the sensor 10 and the sensors 15 a, 15b independently or using a combination. In the event these areindependently measured, the values may be compared and resolved toimprove accuracy, such as by determining the likelihood of eachmeasurement. It may be determined that one of the sensors 10, 15 a, 15 bis more accurate at certain temperature ranges than at other temperatureranges.

As another example, one could assume that the sensors 10 of FIG. 4 arespaced such that properties of the susceptible material 42 are identicalor substantially similar at each location of a first sensor 10 and asecond sensor 10. For example, the thickness and temperature of thesusceptible material 42 at the first sensor 10 may not differ much, ifat all, from the thickness and temperature of the susceptible material42 at the second sensor 10. The processor 26 may determine thethickness, the rate of thickness change, and/or the temperature of thesusceptible material 42 of each of the sensors 10 independently. If, forexample, the first sensor 10 does not vary its electrical property withtemperature while the second sensor 10 does vary with temperature, thenthe processor 26 may determine the amount of change of the electricalproperty due to temperature. The change in the electrical property dueto temperature may then be correlated to a thickness or a change inthickness of the susceptible material 42. Similarly, the sensors 10 maybe used to independently compute the thickness of the susceptiblematerial 42 to compare and improve the measurement. Use of the sensors10 along or within the susceptible material 42 may permit the processor26 to generate a tomographic image or other representation of thetemperature and/or a tomographic image of the thickness (or change ofthe thickness, such as ablation, erosion, corrosion or the like) of thesusceptible material 42.

FIG. 5 illustrates an embodiment of the disclosure of the sensors 10, 15a, 15 b embedded into a thermal protection system 200 of a spacevehicle, such as a heat shield of an aeroshell. The thickness of theaeroshell 200 is defined by a first end 61 and a second end 81. Thesensors 10, 15 a, 15 b may be micro scale sensors fabricated inmaterials that are compatible with the material of the thermalprotection system 200. Any numbers of the sensors 10, 15 a, 15 b, suchas hundreds, are fabricated in the substrate 12 and may be released intothe material prior to final forming of the thermal protection system200. In an embodiment, at least one of the sensors 15 a, 15 b may beused for each of the sensors 10. For example, each of the sensors 10 mayhave at least one of the sensors 15 a, 15 b positioned adjacent thereto.Each of the sensors 10, 15 a, 15 b and/or each of the substrates 12 maybe positioned transversely in strategic locations of the thermalprotection system 200 before final casting, molding, or compression. Inan embodiment, the sensors 10, 15 a, 15 b are positioned so that each ofthe sensors 10, 15 a, 15 b extends along the thickness of the thermalprotection system 200. Each of the conducting lines 30 a, 30 b may beconnected to terminals 20, 20 b as shown in FIGS. 2-4. In the case ofthe thermal protection system 200, the conducting lines 30 a, 30 b areextending out of a cold end, the second end 81, of the thermalprotection system 200. In other words, the first end 61 is generally theend exposed to high temperatures during reentry. The conducting lines 30a, 30 b and the terminals 21 a, 21 b of the sensors 15 a, 15 b aresubsequently connected to an electronic circuit, such as the electriccircuit 50 shown in FIGS. 3 and 4, that is used to generate atomographic image of the surface temperature and recession rate of thethermal protection system 200. The processor 26 may be connected. Theprocessor 26 may be connected to transfer data or information from thethermal protection system 200 to other locations of the space vehicle.

Thermoelectric devices 60 a-60 f may be embedded or otherwise positionedin the thermal protection system 200. It should be understood that thethermoelectric devices 60 a-60 f may be located on and/or may beembedded in the substrate 12. The thermoelectric devices 60 a-60 f (aswell as the other features of the thermal protection system 200) may beused with, in addition to and/or as an alternative to features of theaforementioned embodiments of the disclosure.

The thermoelectric devices 60 a-60 f may scavenge thermal energy of thethermal protection system 200. For example, during reentry to Earth, thespace vehicle and, in turn, the thermal protection system 200 mayencounter extremely high temperatures permitting thermal energy to beconverted to electrical energy by the thermoelectric devices 60 a-60 f.The thermoelectric devices 60 a-60 f may be electrically connected toone or more of the sensors 10, 15 a, 15 b to provide power or current tothe sensors 10, 15 a, 15 b. As shown in FIG. 5, the thermoelectricdevices 60 a-60 f may be positioned at different locations along thethickness of the aeroshell so that ablation of the aeroshell may noteliminate at least some of the thermoelectric devices 60 a-60 f. Thethermoelectric devices 60 a-60 f may generate sufficient power for oneor more of the sensors 10, 15 a, 15 b. The thermoelectric devices 60a-60 f may be in thermal contact with the heat source and thermallyinsulated from a heat sink by a cavity, for example, in the substrate12, or other thermal insulation device in the thermal protection system200.

FIG. 6A illustrates another embodiment of a substrate 110 having a firstsensor 80, a second sensor 85, and a third sensor 90. The substrate 110may be susceptible to corrosion, ablation and/or temperature changes.The substrate 110 may have qualities and features similar to thesubstrate 12. A first sensor 80 may be positioned on and/or etched intothe substrate 110. In an embodiment, the first sensor 80 may besubstantially resistant to temperature changes. For example, theproperties of the first sensor 80 may be substantially constant withrespect to temperature. As a non-limiting example, the first sensor 80may be a capacitor strip providing a decrease in capacitance as thefirst sensor 80 is shortened, such as by ablation, corrosion or thelike. To this end, the first sensor 80 may provide a measurement relatedto ablation or other change in thickness of the substrate 12, 110 and/orthe susceptible material 42. The measurement from the first sensor 80may be a capacitance that relates to a dimension of the first sensor 80,the substrate 12, 110 and/or the susceptible material 42. For example,the measurement may indicate that a length of the first sensor 80 hasdecreased, indicating corrosion or ablation of the susceptible material42.

The first sensor 80 may comprise a first electrode 82 and a secondelectrode 84 as shown in FIG. 6B. A dielectric 83 may separate the firstelectrode 82 from the second electrode 84. The dielectric 83 may be, forexample, an electric insulator or any substance capable of substantiallypreventing electrical charges from passing therethrough. The dielectric83 defines the gap or separation between the first electrode 82 and thesecond electrode 84, which of course may be customized based onapplication. A potential difference may be created across the firstelectrode 82 and the second electrode 84. The surface area of the firstelectrode 82 and the second electrode 84 may decrease as corrosion orablation occurs on the substrate 110 and/or the susceptible material 42.As a result, in an embodiment where the first sensor 80 is a capacitor,the decrease in surface area of the electrodes 82, 84 decrease acapacitance.

Bondpads 86 a, 86 b may be positioned at opposing sides of the firstsensor 80. For example, the bondpads 86 a, 86 b are positioned onopposite sides of the first electrode 82 and the second electrode 84.The bondpads 86 a, 86 b may be made of a material that can beincorporated into, attached to and/or secured to the susceptiblematerial 42. The first bondpad 86 a may be positioned on the firstelectrode 82, and the second bondpad 86 b may be positioned on thesecond electrode 84, as shown in FIG. 6B.

FIGS. 6C and 6D illustrate a cross-sectional view of the second sensor85 and the third sensor 90, respectively. The second sensor 85 and thethird sensor 90 may each have bondpads 86 a-86 d positioned on the firstelectrode 82 and the second electrode 84, respectively. The bond pads 86c and 86 d may be separated a distance from bondpads 86 a and 86 b. Thefirst electrode 82, the second electrode 84 and the dielectric 83 mayextend between the bondpads 86 c, 86 d and the bond pads 86 a, 86 b.

The second sensor 85 may have any number of electrical components 141a-141 f. In the embodiment shown in FIG. 6A, the second sensor 85 may beutilized to determine an ablation, corrosion or other change inthickness of the substrate 110 and/or the susceptible material 42. As aresult, the second sensor 85 may position the electrical components 141a-141 f at predetermined positions representative of the degree ofmeasurement of the thickness of the substrate 110 and/or the susceptiblematerial 42 that is desired or required. For example, the electricalcomponents 141 a-141 f may be positioned such that one of the electricalcomponents 141 a-141 f is separated from the second sensor 85 at eachposition or thickness in which it may be beneficial to determine athickness, ablation or corrosion of the substrate 110 and/or thesusceptible material 42.

For example, the second sensor 85 may be arranged as a resistor laddernetwork where at least a portion of the electrical components 141 a-141f is positioned at rungs of the ladder, for example. The separation,disconnection or destruction of a rung of the ladder network changes theelectrical properties of the second sensor 85. For example, if theelectrical components 141 a-141 f comprise resistors, one less resistormay be connected to the ladder network as each run of the ladder isdestroyed or broken. As each of the electrical components 141 a-141 fare separated (or destroyed) from the second sensor 85, the resistanceof the second sensor 85 may change, such as increase. The change in theelectrical property of the second sensor 85 may be substantially relatedto a change in thickness of the substrate 110 and/or the susceptiblematerial 42. Accordingly, in an embodiment, the electrical components141 a-141 f have a minimal change with respect to temperature. As aresult, the second sensor 85 may output or measure the ablation orcorrosion of the substrate 110 and/or susceptible material 42 withminimal effects on temperature.

The third sensor 90 may be temperature dependent such that an output ormeasurement of the third sensor 90 changes based on temperature. Forexample, the third sensor 90 may comprise one or more electricalcomponents 141 a-141 f that change with respect to temperature. In anembodiment, at least a portion of the electrical components 141 a-141 fmay comprise resistors that change resistance based on temperature. Thesecond sensor 85 may be arranged such that one or more of the electricalcomponents 141 a-141 f are separated upon ablation or corrosion of thesubstrate 110. For example, the third sensor 90 may be arranged in aladder configuration as shown in FIG. 6A where one or more of theelectrical components 141 a-141 f is separated based on ablation orcorrosion of the substrate 110. In an embodiment where the third sensor90 primarily measures temperature and the second sensor 85 primarilymeasures ablation, the third sensor 90 may have less rungs in the laddernetwork and/or have electrical components that are more temperaturedependent.

The third sensor 90 may have any number of electrical components 141a-141 f that may be arranged at predetermined distances along the thirdsensor 90 in order to cause separation of one or more of the electricalcomponents 141 a-141 f. Accordingly, any degree of ablation or corrosionmay be monitored by positioning the electrical components 141 a-141 f atpredetermined positions.

In an embodiment, the separation (or destruction) of one or more of theelectrical components 140 a-140 h from the second sensor 85 changes theelectrical property as does changes in temperature of the second sensor85. As there may be a number of variables in the changes in the outputor measurement of the second sensor 85, the measurement or output of thesecond sensor 85 may be compared to the output or the measurement fromthe first sensor 80 and the third sensor 90, and vice versa. In anembodiment where the first sensor 80 is a capacitor strip, the firstsensor 80 may provide a relatively accurate measurement of the thicknessor the susceptible material 42. This measurement of the thickness of thesusceptible material 42 may be used to determine or predict an amount ofchange in the electrical properties of the second sensor 85 and thethird sensor 90 due to the change in thickness. For example, in anembodiment where the second sensor 85 is a resistor ladder sensor, thechange in thickness may destroy or otherwise electrically separate oneof the resistors causing a change in the electrical property, such as achange in resistance, of the second sensor 85.

As the second sensor 85 and the third sensor 90 may be sensitive totemperature such that electrical properties of the second sensor 85 andthe third sensor 90 change with temperature, the electrical propertiesor measurements of the second sensor 85 and the third sensor 90 may becompared. However, the electrical properties of the second sensor 85 andthe third sensor 90 may also change due to ablation or corrosion of thesusceptible material 42. In order to determine an amount of change ofthe electrical properties of the second sensor 85 and the third sensor90 due to temperature, the measurements may be compared in addition tothe measurement of the first sensor 80. The comparison may be ananalysis, applying a logic, algorithm or the like using a processor,such as the processor 26.

In an embodiment the first sensor 80, the second sensor 85 and the thirdsensor 90 may be incorporated into a network whereby numerous sets ofthe first sensor 80, the second sensor 85 and the third sensor 90. In anembodiment, each set of the sensors 80, 85, 90 may be incorporated intoa network or circuit. Each set or each of the sensors 80, 85, 90 mayoperate similar to a node of a circuit and/or may be connected to amultiplex. The multi-plex may communicate with and interrogate with eachsensor and may be addressable to each sensor by using an address of eachsensor. A demultiplex may be used to display the sets of sensors 80, 85,90 as an array on a display. For example, the display may show atemperature and thickness at each node, such as at easch set of thesensors 80, 85, 90.

FIGS. 7A-7G illustrate an embodiment of fabricating and/or manufacturingthe first sensor 80, the second sensor 85 and the third sensor 90. Itwill also be appreciated by those having ordinary skill in the art thatthe embodiment of fabrication and manufacturing methods FIGS. 7A-7G maybe utilized for the sensors 10, 15 a-15 e. It should also be appreciatethat the description of the manufacturing and fabrication process is forexplanatory purposes, and the present disclosure should not be deemed aslimited to this fabrication process or any specific fabrication process.

FIG. 7A illustrates a side view of a wafer 300 which may be utilized inan embodiment of the disclosure. The wafer 300 may be, for example, ahigh resistivity p-type substrate, or a semi-insulating substrate. Thewafer 300 of the disclosure should not be deemed as limited to any typeof substrate dimensions, but at least in an embodiment may have athickness of approximately 200 microns. The wafer 300 may be cleanedusing a solvent to remove organics, and then the wafer 300 may be dippedin a chemical solution to remove any trace metals, such as a solution ofhydrogen peroxide and sulfuric acid. As a non-limiting example, thewafer 300 is dipped into a chemical solution comprising substantiallyequal volumes of hydrogen peroxide and sulfuric acid for 15 minutes. Thewafer 300 may be rinsed, such as by use of de-ionized water. Inaddition, the wafer 300 may be blown dry with a gas, such as nitrogen.

FIG. 7B illustrates deposition of a layer 302 on the wafer 300. Thelayer 302 may be positioned on the wafer 300 by any process known tothose having ordinary skill in the art, including microfabricationtechniques, such as sputtering. In an embodiment, the layer 302 may bemetallic, such as gold. The layer 302 may then be annealed in a gas,such as nitrogen. For example, the layer 302 may be annealed in nitrogenat 200 degrees Celsius for about thirty minutes. Photoresist or otherlight sensitive material may be applied on the layer 302 and spun, suchas at 3000 revolutions per minute for thirty seconds. Next, the wafer300 with the layer 302 may be baked at 90 degrees Celsius in a gas, suchas nitrogen ambient for a duration, such as five minutes. Next, abondpad mask is placed over the photoresist and exposed under ultraviolet light, such as for ten seconds. The wafer 300 may then beinserted in a developer for about one and a half minutes to develop thephotoresist. The wafer 300 may be rinsed in de-ionized, blow dried, andbaked again for five minutes in nitrogen ambient at 90 degrees Celsius.

FIG. 7C illustrates that the layer 302 is etched in 10:9:1 volume ratioof H₂O:HCl:HNO₃ at 40 degrees Celsius until a portion of the layer 302is dissolved, leaving only the bondpad gold under the protectivephotoresist. Next, the photoresist is dissolved in acetone and the waferand rinsed in de-ionized water. The layer 302 may be one of the bondpads86 a-86 d as shown in FIGS. 6B-6D. FIG. 7D illustrates a top view of thewafer 300 of FIG. 7C.

A layer 304 may be deposited on the wafer 300 and the layer 302 as shownin FIG. 7E by any microfabrication technique, such as sputtering. Thelayer 304 may be an electrode, such as a metal, for example platinum,and may have any dimension, but in this example is approximately 400nanometers. A silicon or other bonding agent may be deposited on thelayer 304 to promote sealing with the layer 306. Next, the layer 306 isdeposited by any known microfabrication technique. The layer 306 may bea dielectric material, such as silicon dioxide. The layer 306 may, in anembodiment, be the dielectric 83 as shown in FIGS. 6B-6D.

A layer of silicon or other bonding agent is deposited on the layer 306to promote adhesion. The silicon may be a thin layer, such as 10nanometers. The layer 308 is then deposited on the layer 306, as shownin FIG. 7F by any known microfabrication technique. The layer 308 may bemade of an electrode, such as a metal, for example, platinum. The layer308 in this embodiment is about three nanometers. The layer 308 may bethe electrode 82 as shown in FIGS. 6B-6D.

Next, a layer 310 is deposited on the layer 308 using a knownmicrofabrication technique, as shown in FIG. 7G. The layer 310 may be ametallic layer, such as gold. The layer 310 may be about 200 nanometers.Photoresist or other light sensitive material may be applied on thelayer 310 and spun, such as at 3000 revolutions per minute for thirtyseconds. Next, the wafer 300 with the layer 302 may be baked at 90degrees Celsius in a gas, such as nitrogen ambient for a duration, suchas five minutes. Next, a bondpad mask is placed over the photoresist andexposed under ultra violet light, such as for ten seconds. The wafer 300may then be inserted in a developer for about one and a half minutes todevelop the photoresist. The wafer 300 may be rinsed in de-ionized, blowdried, and baked again for five minutes in nitrogen ambient at 90degrees Celsius.

FIG. 8A illustrates that the layer 310 is etched in 10:9:1 volume ratioof H₂O:HCl:HNO₃ at 40 degrees Celsius until a portion of the layer 310is dissolved, leaving only the bondpad gold under the protectivephotoresist. Next, the photoresist is dissolved in acetone and the waferand rinsed in de-ionized water. The layer 310 may be one of the bondpads86 a-86 d as shown in FIGS. 6B-6D. FIG. 8B illustrates a top view of thewafer 300 of FIG. 8A.

A layer of aluminum may then be deposited on the surface by sputteringor e-beam technique. Photoresist is applied on the aluminum surface andspun at 3000 revolutions per minute for about thirty seconds. Afterapplying the photoresist, the wafer 300 may be baked at 90 degreesCelsius in a gas, such as nitrogen ambient for about five minutes. Asensor element mask may be positioned over the photoresist and exposedunder ultra violet light for about ten seconds. The wafer 300 is theninserted in a developer for about one and half minutes to develop thephotoresist. The wafer 300 is rinsed in de-ionized, blow dried, andbaked again for about five minutes in gas, such as nitrogen ambient, atabout 90 degrees Celsius.

The aluminum layer may be etched using any known etching technique aswill be appreciated by those of ordinary skill in the art. For example,the aluminum layer may be etched chemically using phosphoric acid atabout 50 degrees Celsius under the field aluminum dissolvessubstantially or preferably completely, leaving the aluminum under theprotective photoresist. To remove the photoresist, acetone may be usedto dissolve the photoresist. Then, the wafer 300 may be reinsed inde-ionized water. Using the aluminum as the etch mask, the areas notprotected by the aluminum are etched by reactive ion etching method tothe wafer 300 as shown in FIG. 8C. The residual aluminum surviving thereactive ion etching is dissolved in phosphoric acid at about 50 degreesCelsius, followed by rinsing with de-ionized water and blowing dry witha nitrogen gas.

The underside of the wafer 300 is etched by reactive ion etching to thinthe wafer 300 to at least 100 microns. The front side of the wafer 300is mounted to a carrier substrate with a thin layer of photoresist andbacked at 90 degrees Celsius in nitrogen ambient for about 30 minutes. Aseed layer of nickel may be deposited on the backside. Photoresist isapplied to the nickel see layer and spun at 3000 revolutions per minutefor about 30 seconds. This is followed by baking at 90 degrees Celsiusin nitrogen ambient for about five minutes. Next, a backside contactmask is placed over the photoresist and exposed under ultra violet lightfor about ten seconds. The wafer 300 is then inserted in a developer forabout one and a half minutes to develop the photoresist. The wafer 300is rinsed in de-ionized, blow dried, and baked again for five minutes innitrogen ambient at 90 degrees Celsius.

Nickel may be electroplated on the portion of the nickel seed layer notprotected by the photoresist. The dissolution of the photoresist may beaccomplished with acetone and subsequent rinsing in de-ionized water.Using the thick nickel as an etch mask, the backside of the wafer 300 isetched to expose the layer 302, which is the first bondpad layer. Thethickness of the electroplated nickel should be such that it iscompletely etched just before the bondpads are exposed. This permits acomplete removal of the nickel and the etching continues on the entirebackside of the wafer 300 until the bondpads are exposed. The wafer 300is un-mounted from the carrier by immersing in acetone.

During re-entry of the space vehicle into Earth, the thermal protectionsystem 200 may encounter thermophysical effects (shock, radiation,ionization, non-equilibrium chemistry) that combine to lead to itsablation and subsequent recession. As the thermal protection system 200recedes, the sensors 10, 15 a, 15 b, 80, 85, 90 also recede. Thisresults in a change in an electrical property of the sensors 10, 15 a,15 b, 80, 85, 90 such as a change in the capacitance or resistance ofthe sensors 10, 15 a, 15 b, 80, 85, 90. For example, if the sensor 10,85, 90 is arranged in a ladder network, then the rate of breaks of thesteps of the ladder network (or the rate at which one of the electricalcomponents separates from the other electrical components) directlyrelates to the recession rate of the thermal protection system 200. Incases where the recession across the thermal protection system 200 isasymmetric, such effect would be reflected on the sensor 10, 15 a, 15 b,80, 85,90 that corresponds to that location.

In an embodiment, the system and method disclosed herein may be utilizedto enhance temperature and recession measurements and/or maysignificantly aid in optimizing the geometric shape of the susceptiblematerial 42, such as an aero-shell of a space vehicle. Instead of havinga few measurement plugs as in prior art devices, hence few measurementlocations, the present disclosure provides a system and methodpermitting temperature and ablation recession sensors 10, 15 a, 15 b,80, 85, 90 to be arrayed over a larger surface area, such as a largersurface area of the thermal protection system material 200. Each sensor10, 85,90 may comprise one or more resistors, such as a micro-millimeterscale resistor ladder network, fabricated in materials that arecompatible with the susceptible material. The resistor ladder networkmay be fabricated in the substrate 12 and released. Each resistor laddernetwork is positioned along the thickness of the susceptible material42. The sensors 15 a, 15 b, 80 which may be capacitor strips, may bepositioned adjacent each of the sensors 10 to determine a change in thethickness of the susceptible material (recession rate).

1. A system comprising: a material susceptible to corrosion or ablationand having a thickness defined between a first end and a second end; afirst sensor and a second sensor on the material and extending along thethickness adjacent the first end and toward the second end, the firstsensor having an electrical component that changes an electricalproperty of the sensor as temperature of the material changes, andwherein the second sensor has an electrical property that issubstantially constant with respect to changes in temperature of thematerial, an electrical circuit coupled to the first sensor and thesecond sensor to determine the change in the electrical property of eachof the first sensor and the second sensor; and a processor determining athickness and a temperature of the material based on the change of theelectrical property of the first sensor and the second sensor.